This invention relates to an apparatus for controlling exhaust gas discharged from an internal combustion engine, and more particularly to an apparatus which is used for control of the exhaust gas in a closed loop in which the air-fuel ratio of the gas mix in the carburetor is so controlled by means of signals from an exhaust gas sensor that the air-fuel ratio is always maintained close to a theoretical air-fuel ratio in order to effectively utilize a (exhaust gas purification) cleaning function of a 3-way catalyst, and which is adapted to supply, during the idle mode and during the cold mode of the engine, secondary air to the gas mix in a region between the exhaust gas sensor and the 3-way catalyst.
The 3-way catalyst has been used as means for simultaneously reducing the contents of three harmful components, NOx, HC and CO contained in the exhaust gas discharge from internal combustion engines. For utilizing the 3-way catalyst in the most efficient manner, it is necessary to precisely control the air-fuel ratio of the gas mix being supplied to an engine so that the air-fuel ratio of the gas mix may be maintained as close to the theoretical air-fuel ratio as possible. For this purpose, an air-fuel ratio control apparatus of feedback type has been developed. With this conventional control apparatus, however, there are some cases where the cleaning function of the 3-way catalyst cannot be effectively obtained. For example, in the case of a transition from the idle mode to the acceleration mode or during the shifting of the gears, the control is unable to follow a rapid variation in the air-fuel ratio. In the case of a cold start or in the case of a long time idling, the temperature of the exhaust gas sensor decreases resulting in an increased internal impedance, thus making the normal control impossible to be obtained. As a result, large amounts of the harmful components, especially CO, may be discharged in the air. For preventing this, a system has been proposed wherein, at the time of idle or gear shift, for example, secondary air is supplied upstream of the 3-way catalyst to the exhaust gas to dilute the gas so that the 3-way catalyst can function in an oxidizing region, thereby to effectively eliminate CO. As means for supplying such secondary air, an air supply device, which comprises an air pump of a vane type and an electromagnetic three-way valve, has been employed. In this system, however, since the air pump is driven through rotation of the engine, a part of the output of the engine is consumed in the air pump, resulting in a decreased effective output of engine. Especially in a region of the running mode which requires the maximum output, a decrease in the effective output is significant because of an increased pressure of the exhaust gas and an increased load put on the air pump.